1. Field of the Invention
The present invention relates to a pilot signal detection circuit, and more particularly to a pilot signal detection circuit for detecting a pilot signal indicating whether broadcasting is sound multiplex broadcasting or stereophonic broadcasting.
2. Related Art
A two-carrier sound multiplex system has been known as a sound multiplex system. FIG. 2 shows frequency spectrums of the two-carrier sound multiplex system. A sound signal is frequency-modulated by using two IF signals as the carrier and then transmitted. The frequency spectrums shown in FIG. 2 indicates spectrums after the IF signals are FM-detected.
As shown in FIG. 2, a first sound (voice) signal includes a (L+R) signal or a main sound signal. The second sound (voice) signal includes a (L-R) signal or a sub sound signal together with a pilot signal indicating a mode type of the transmitted signal (a stereophonic broadcasting signal or a sound multiplex broadcasting signal). A signal whose frequency is 3.5 times a horizontal sync signal (fH) of an image signal is used as the carrier. In case of the stereophonic broadcasting, the pilot signal is obtained by amplitude-modulating the carrier with a signal having a frequency of fH/133 (approx. 117.5 Hz). In case of the two-carrier sound multiplex system, the pilot signal is obtained by amplitude-modulating the carrier with a signal having a frequency of fH/57 (approx. 274.1 Hz).
A broadcasting mode is determined by detecting the frequency (fH/133 or fH/57) of the amplitude-modulating signal from the pilot signal and in accordance with the detected frequency.
A pilot signal detection circuit shown in FIG. 3 has been known. In FIG. 3, the second sound signal as shown in FIG. 2 is input to a band-pass filter (BPF) 31 which extracts the pilot signal of 3.5 fH. The extracted pilot signal is demodulated by an AM demodulator 32.
The frequency of the AM-demodulated signal is 117.5 Hz in the stereophonic broadcasting and 274.1 Hz in the two-channel sound multiplex broadcasting. Therefore, the AM-demodulated signal is supplied to a BPF 33 whose pass band having a center frequency of 117.5 Hz and a BPF 34 whose pass band having a center frequency of 274.1 Hz. The output signals from the BPFs 33 and 34 are rectified, smoothed and converted into a DC voltage by a pilot detector 35. Thereafter, the obtained DC voltage is compared with a predetermined threshold level so that the broadcasting mode corresponding to the DC voltage exceeding the threshold level is determined as the current broadcasting mode. More specifically, if the output signal from the BPF 33 is detected, the stereophonic broadcasting is determined, while if the output signal from the BPF 34 is detected, the two-channel sound multiplex broadcasting is determined.
Another conventional pilot detection circuit described in Kokai (Laid Open of Japanese Patent Application) No. 2-105784 is shown in FIG. 4.
In FIG. 4, the second sound signal is input to the BPF 41 which extracts the pilot signal of 3.5 fH and neighboring upper and lower side-band components (see FIG. 2). The output signal from the BPF 41 is input to a side-band detector 42 to detect the side bands. A pilot detector 43 detects the broadcasting mode based on the detected side bands.
A side-band detector 42 detects the side bands by using a signal of 62.5 KHz supplied from a switch 44. The switch 44 supplies one of the following signals to the side-band detector 42: (i) a signal of 62.5 KHz synchronized with fH generated by a PLL circuit comprised of a VCO 46 and a phase comparator 45, (ii) an externally supplied signal of 62.5 KHz, and (iii) a signal of 62.5 KHz obtained by 1/64 frequency-dividing a signal of 4 MHz from a 4 MHz-crystal oscillator 47 by a 1/64 frequency divider 48.
According to the circuit shown in FIG. 3, the detection accuracy of the pilot signal depends mainly on a selection characteristic of each of the BPFs 33 and 34. For this reason, the selection characteristic of the BPFs 33 and 34 must be improved in order to improve the detection accuracy of the pilot signal. More specifically, the quality factor Q of BPFs 33 and 34 must be large.
When the Q values of the BPFs 33 and 34 are set to be high, however, the variations of the center frequencies of the pass bands of the BPFs 33 and 34 must be set small. Accordingly, it is difficult to manufacture the BPFs 33 and 34. Furthermore, the BPFs 33 and 34 will be costly even if they are manufactured. Further, if the BPFs 33 and 34 are integrated into an IC, it is difficult to set the Q values of BPFs 33 and 34 high.
On the other hand, according to the conventional circuit shown in FIG. 4, three-types of reference signals (62.5 KHz signals in an example of FIG. 4) must be generated in order to detect the side bands, resulting in complicated circuit arrangement. Particularly, the arrangements of the crystal oscillator and the PLL circuit become complicated.
Further, the detection circuit shown in FIG. 4 must have the external reference signal to be supplied to the side-band detector 42. For this reason, the detection performance will be poor in weak electric field (when a received signal is weak and a level of the second input signal is low).